1. Field of the Invention
The present invention generally relates to an optoelectronic communication system, and more particularly to an optoelectronic communication system in a turbulent medium that employs a receiver having an array of optical fibers and photodetectors.
2. Description of the Prior Art
The telecommunications industry is switching rapidly to a hybrid platform which utilizes both electronics and photonics to increase the operating bandwidth of the communication system. Today""s communication systems consist of optical fiber networks, fiber amplifiers, optical diode transmitters, and high speed semiconductor receivers. This architecture works well in the confines of optical fibers, however, free space propagation of these signals which is necessary for remote applications have problems in matching optical fiber network bandwidths.
Free space propagation of the signal through the atmosphere, water or other turbulent media introduces fluctuating distortions and aberrations. These fluctuating distortions prevent focusing the signal beam onto the small area high speed detectors typically utilized in optical communication systems. An optical transmitter, for example, including a 0.05 m collection dish with a focal length of 1 m would concentrate the light into a diameter of 200 xcexcm for a 100 times diffraction limited optical communication beam. Aberrations similar to this could be incurred by atmospheric propagation at a wavelength of 1.5 xcexcm. This area is approximately 300 times larger than that of the high speed semiconductor photodiode detectors employed in conventional communication systems. One approach is to correct for the fluctuating distortions with adaptive optics or phase conjugation techniques in order to obtain a near diffraction limited signal beam which allows focusing onto a small high speed receiver. These techniques suffer from the slow response time, limited phase front correction or high signal intensities required for efficient conjugation. Another approach is to use a large area detector in the optical receiver so that a significant fraction of the distorted signal beam can be collected by the receiver. This method has many advantages but has proven difficult to implement since the detector temporal response and the detector area are often inherently coupled.
What is needed, therefore, is an optoelectronic communication system with integrated high speed small area detectors in an optoelectronic receiver architecture which provides a large effective area receiver while it retains the large intrinsic bandwidth of the individual detector elements.
In addition, it is desirable for the optoelectronic receiver to develop time compensated electrical signals.
Transmission of an optical signal through a turbulent media, such as the atmosphere, produces a fluctuating spatial intensity pattern due to optical distortions and aberrations. With respect to FIGS. 1A, 1B, and 1C, three views are shown of an optical signal being transmitted through a turbulent media at three instants of time, t1, t2 and t3, respectively. These time varying distortions make it impossible to focus the signal beam onto a single small high speed detector illustrated by the numeral 10 typically utilized in optical communication systems. The present invention involves collecting either a large enough subarray of the distorted signal (shown by the numeral 12) or the entire distorted signal (shown by the numeral 14 and encompassing the periphery in FIGS. 1A, 1B and 1C) with a detector array. The collected signal is invariant to the fluctuating distortions, thereby eliminating problems in free space propagation of optically transmitted high bandwidth signals.
The invention involves combining high speed small area photodetectors utilizing an optoelectronic interconnect, preferably with a time compensated reading (probe) beam methodology into a receiver to increase the effective detector area while maintaining the temporal response (high bandwidth). The large effective area detector captures all of the signal beam thereby eliminating problems with regard to optical signal transmission through turbulent media. This detector array directly measures temporal pulses, however, it is applicable to both temporally, phase or frequency encoded signal sources. Phase encoded information would require an optical interferometer before the photodetector array to convert the phase modulation into temporal information.
In another aspect, this invention also involves using a separate transmission beam and reading (probe) beam. A decoupled architecture allows each of the optical wavelengths to be optimized for their own individual function. For example, the transmission wavelength can be chosen to increase signal throughput through the turbulent media while the reading beam could allow for dispersion free fiber propagation, the highest photodetector efficiency or the fastest temporal response.
The described construct utilizes optoelectronic sampling to combine the electric fields from each of the photodetectors in the array. Optoelectronic sampling utilizes an electro-optic material (for example LiTaO3, LiNiO3, GaAs, or birefringent polymers) with the birefringent axis properly oriented with respect to the electric field and an optical probe beam. The probe beam can either be continuous wave to provide real time signal processing or pulsed (i.e., mode-locked, Q-switched) to provide a high speed time gated detector capability. Depending on the orientation of the electric field and optical probe beam polarization relative to the birefringent axis of the electro-optic crystal this field can be made to induce a time dependent polarization rotation or a time dependent phase change on the optical probe beam. This change varies with the electric field strength and therefore with the light intensity incident on the photodetector. Typically the electro-optic response can be made to be either quadratic (homodyne configuration) or linear with the electrical field strength (heterodyne configuration) depending on the probe beam configuration. The temporal resolution of electro-optic sampling is limited by the propagation delay of the optical probe beam across the region where the electric field exists and by the optical phonon band which is materially dependent and typically occurs in the 3-10 THz range. This corresponds to 100-300 fs, respectively, for a full width period at half maximum frequency. For the small area photodetectors having a diameter of approximately 15 xcexcm that are utilized in today""s high bandwidth communications systems, the propagation delay across individual elements would be of the order of 100 fs. Depending upon the temporal response of the individual photodetector elements, the effective area of the photodetector array and the required temporal response (bandwidth) of the array different time compensation configurations can be adopted for the receiver.
Calculations of the effective areas of photodetector arrays for the following architectures, assuming an intrinsic detector element response time of 5 ps and a detector array requirement of less than 6 ps temporal resolution indicate that (1) Without time compensationxe2x80x94the effective detector area can be increased by 33 times; (2) For one dimensional time compensation where an optical element is utilized to compensate the signal across each row (or column) of fibers so one element of each row (or column) is time coincident with the probe beam, the effective detector area can be increased by 1000 times. The optical element could be an optical wedge or stacked optical plates; and (3) For two dimensional time compensation (i.e., element by element time compensation) where an optical element is utilized to compensate the signal at each photodetector element, there is no limit to the effective receiver area with regard to temporal resolution. This two dimensional optical element could be a stepped optical wedge or the time compensating could be accomplished by the preferred method of tailoring the optical fiber lengths themselves.
The temporal resolution of the element by element time compensated device is currently limited by the speed of today""s photodetector elements, i.e., 5 ps, 60 GHz. However, as this speed increases either through advances in fabrication techniques, use of other materials or new configurations, the electro-optic sampling interconnect will be capable of supporting temporal resolution of the order of 100 fs which corresponds to data rates of 10 THz.
There are a number of techniques which can be utilized to increase the operational bandwidth even further. Wavelength multiplexing akin to that used in fiber optic systems can also be applied to increase this receiver""s operational bandwidth. A spectrally selective optical element, such as a Bulk or a fiber Bragg grating, an acousto-optic or electro-optic deflector, a prism, or an interference filter would be utilized to route the different wavelengths to a different detector array. The receiver bandwidth would therefore increase linearly with the number of signal wavelengths. Fiber based wavelength multiplexing, for example, has expanded bandwidths by factors of greater than 100 times.
In this invention, amplitude modulation of the reference beam could be utilized along with the temporal or phase modulation of the signal beam to encode information. For example, with a signal to noise level of greater than 64:1, 6 bits of information could be encoded instead of 1 bit per pulse allowing an increase in the total bandwidth by a factor of 6. The detector effective area will ultimately be limited by signal to noise constraints instead of temporal resolution. There are a number of methods to increase the signal to noise ratio. For example, the signal beam can be optically amplified in a solid state gain media before entering the fiber array, the fibers themselves could serve as optical amplifiers, or the detector array can be thermally cooled.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention that provides generally an optoelectronic communication system for use with an optical signal that passes through a turbulent environment comprises an optical transmitter for transmitting an optical signal, and an optoelectronic receiver. The optoelectronic receiver comprises an optical reflector for collecting the optical signal and for propagating a plurality of portions of it, a plurality of first optoelectronic detector means, each being responsive to a selected portion of the optical signal and operative to develop a plurality of electrical signals, each representing a portion of the information, a probe laser for generating an optical probe beam, means responsive to the plurality of electrical signals and operative to change a characteristic of the optical probe beam corresponding to the information, and a second detector responsive to the changed characteristic and operative to develop an output or resultant signal representative of the information contained in the received optical signal. The output signal may be electrical or optical.
In another aspect the present invention provides an optoelectronic receiver for receiving an optical signal and includes high speed small area detectors arranged in a time compensated architecture.